Custom Map Makers

I am a super noob and don't know how to make tunnel/boxes you can walk inside of. I am wanting to make underground passages / building connectors but am in need of a decent guide. Can you point me in the right direction?

Also I should add that this is going to be a strictly ts map, and the layout shouldn't be symmetrical.

I realize that there's a big open space in the middle but I kind of wanted it to be the concrete yard in prison. I was also thinking about just placing a bunch of objects in the yard to have things to hide behind in fire fights. I can add more routes around the backs of building etc. not sure if you...

Not sure if anyone has had this idea before but sorry if I'm copying you! Very new to mapping and am looking for tutors and people to help me create this map. Here is a snapshot of my layout so far. http://i.imgur.com/KprgA.png http://i.imgur.com/i2Tc9.png I'm a noob to mapping so please be patient ...